1. Field of the Invention
This invention relates to a method and adhesive for bonding orthodontic appliances such as brackets to a patient's teeth. More specifically, the present invention relates to an indirect bonding method wherein the ultimate position of the appliance on the teeth is determined in part by use of a stone model and transfer tray, and wherein the adhesive provides specific advantages for carrying out the method.
2. Description of the Related Art
Orthodontic treatment involves movement of malpositioned teeth to orthodontically correct locations. During treatment, tiny appliances known as brackets are often fixed to the patient's teeth, and an arch wire is placed in a slot of each bracket. The arch wire forms a track to guide movement of the teeth to desired locations. Ends of the arch wire are often connected to buccal tubes that are in turn secured to the patient's molar teeth. The brackets, buccal tubes and arch wires are commonly referred to as "braces".
Orthodontic brackets are often bonded directly to the patient's teeth by an adhesive. In one technique, a small quantity of adhesive is placed or "buttered" on the base of each bracket and the bracket is then placed on a selected tooth. Next, the bracket is shifted slightly on the tooth as may be needed to bring the bracket to a desired location on the tooth. Once the adhesive has hardened, the bracket is bonded to the tooth with sufficient strength to withstand subsequent forces as orthodontic treatment progresses.
Many practitioners use adhesives known as chemical curing adhesives in the direct bonding technique described above. Chemical curing adhesives are often supplied in two initially separate components that, once mixed together, react and harden to adhere the bracket to the tooth. Typically, the components are mixed together and a small quantity of the mixture is placed on the back of the bracket prior to placing the bracket on the tooth.
However, the direct bonding method as described above when carried out using a chemical curing adhesive is often considered unsatisfactory because the adhesive may unduly harden before the practitioner has had an opportunity to shift the bracket to its precise, intended location on the patient's tooth. For example, if the procedure is interrupted after the components are mixed together, the adhesive may become too stiff to permit the bracket to be shifted across the surface of the tooth. Such an occurrence is preferably avoided, since the ultimate position of the tooth in the oral cavity as treatment progresses is determined to a significant extent by the precise location of the bracket on the tooth. Moreover, the practitioner must take care not to disturb the bracket during the time the adhesive hardens, which may be on the order of 1.5 minutes.
The development of light-curable adhesives for orthodontic treatment has provided many advantages when used in the direct bonding technique described above. Light-curable adhesives include a photoinitiator that initiates the curing reaction once the adhesive is exposed to a sufficient amount of light. Moreover, light-curable adhesives are premixed by the manufacturer and need not be mixed in the orthodontist's office before use.
A common method of using light-curable adhesives for direct bonding includes the steps of placing a small quantity of the adhesive on the base of the bracket and then placing the bracket on the patient's tooth. The practitioner then shifts the bracket on the tooth as may be needed to move the bracket to its intended location. Once the bracket is in its precise, intended location, light from a dental curing unit is directed toward the adhesive for a time period sufficient to satisfactorily cure the adhesive. As can be appreciated, light-curable adhesives are an advantage in that the adhesive does not normally unduly harden until the practitioner has had an opportunity to move the bracket to its intended location and then direct light from the curing unit toward the adhesive.
While the direct bonding techniques described above are in widespread use and are considered satisfactory by many, there are shortcomings that are inherent with such techniques. For example, access to the surfaces of malposed teeth may be difficult. In some instances, and particularly in connection with posterior teeth, the practitioner may have difficulty seeing the precise position of the bracket relative to the tooth surface. Another problem with the above described techniques concerns the significant length of time needed to carry out the procedure of bonding a bracket to each individual tooth, which is a nuisance both to the patient as well as to the practitioner. The risk of moisture contamination from the patient's saliva also increases as the time increases that the patient is occupying the chair. The above factors may also unduly impair the accuracy of placement of the brackets on the teeth and/or increase the chances that the ultimate adhesive bond will not have sufficient strength to retain the brackets on the teeth during the course of orthodontic treatment.
Indirect bonding techniques avoid many of the problems noted above. In general, indirect bonding techniques in the past have involved the use of a transfer tray having a shape that matches the configuration of at least part of one of the patient's dental arches. A set of brackets are releasably connected to the tray at certain, predetermined locations. Once adhesive is applied to the base of each bracket, the tray is placed over the patient's teeth until such time as the adhesive hardens. Next, the tray is detached from the teeth as well as from the brackets, with the result that all of the brackets previously connected to the tray are now bonded to their respective teeth at their intended, predetermined locations.
In more detail, one method of indirect bonding includes the steps of taking an impression of each of the patient's dental arches and then making a replica plaster or "stone" model from each impression. A sealing solution (such as Liquid Foil brand sealing solution, from 3M) is applied to the stone model and allowed to dry. If desired, the teeth of the model can be marked with a pencil to assist in placing the brackets in ideal positions.
Next, the brackets are bonded to the sealed stone models. Optionally, the bonding adhesive can be a chemical curing adhesive (such as Concise brand adhesive from 3M) or a light-curable adhesive (such as Transbond XT or Transbond LR adhesive, from 3M). Optionally, the brackets may be adhesive precoated brackets such as described in U.S. Pat. Nos. 5,015,180, 5,172,809, 5,354,199 and 5,429,299.
A transfer tray is then made by placing matrix material over the model as well as over the brackets on the model. For example, a plastic sheet matrix material may be placed over the model and brackets and then heated in an oven under vacuum. As the plastic sheet material softens and as air in the oven is evacuated, the plastic sheet material assumes a configuration that precisely matches the shape of the replica teeth of the stone model and adjacent brackets.
The plastic material is then allowed to cool and harden to form a tray. The tray and the brackets (which are embedded in an interior wall of the tray) are then detached from the stone model and sides of the tray are trimmed as may be desired. Once the patient has returned to the office, a quantity of adhesive is placed on the base of each bracket and the tray with the embedded brackets is then placed over matching portions of the patient's dental arches. Since the configuration of the interior channel in the tray closely matches the respective portions of the patient's dental arches, each bracket is ultimately positioned on the patient's teeth at precisely the same location that corresponds to the previous location of the same bracket on the stone model.
Both light-curable adhesives and chemical curing adhesives have been used in indirect bonding techniques to secure the brackets to the patient's teeth. If a light-curable adhesive is used, the tray is preferably transparent or translucent. If a two-component chemical curing adhesive is used, the components can be mixed before application to the brackets, or one component may be placed on each bracket base and the other component may be placed on the tooth surface. In either case, placement of the tray with the embedded brackets on corresponding portions of the patient's dental arches enables the brackets to be bonded to the teeth as a group using only a relatively short amount of time that the patient is occupying the chair in the operatory. With such a technique, individual placement and positioning of each bracket in seriation fashion on the teeth is avoided.
While it is apparent that the use of indirect bonding techniques can greatly facilitate precise placement of brackets on the patient's teeth and shorten the amount of time that the patient is in the operatory, there is a need in the art to improve current indirect bonding techniques in order to optimize use of the practitioner's time. For example, conventional techniques using commercially available chemical curing orthodontic adhesives are somewhat time consuming in that the practitioner is often required to firmly press the transfer tray with the embedded brackets against the patient's teeth for at least two minutes and then wait an additional five minutes before removing the tray from the mouth to ensure that the adhesive has cured to a degree sufficient to hold the brackets firmly on the tooth as the tray is released from the brackets and the teeth. Some conventional light-curable adhesives cure somewhat sooner, but require the use of a dental light curing unit and the use of subsequent disinfection and/or sterilization procedures before using the light curing unit with another patient.